1. Field of the Invention
In an optical pickup apparatus which can be used in common for two or more kinds of optical recording media, the present invention relates to a diffraction type lens which can efficiently converge respective irradiation light beams onto their corresponding optical recording media when the irradiation light beams have respective wavelengths different from each other according to the kinds of optical recording media, and an optical pickup apparatus using the same.
2. Description of the Prior Art
In recent years, various kinds of optical recording media have been under development, and optical pickup apparatus which can carry out recording and reproducing while using a plurality of kinds of optical recording media in common have been known. For example, a system which carries out recording and reproducing of DVD (digital versatile disc) and CD-R (recordable optical disc) by using an optical pickup apparatus has been known.
In such two kinds of optical recording media, for example, visible light at about 635 nm is used for DVD in order to improve the recording density, whereas near-infrared light at about 780 nm is required to be used for CD-R since it has no sensitivity for light in the visible region. An optical pickup apparatus which can be used in common for both of them is based on a dual-wavelength beam type which uses two light beams having wavelengths different from each other.
When the disc thickness value differs between the above-mentioned two kinds of optical recording media, it is necessary for the individual wavelengths of light for reproducing or recording in such an optical pickup apparatus to have converging actions different from each other.
For responding to such a requirement, a system in which two objective lenses having converging actions different from each other are made switchable depending on the optical recording media subjected to reproducing or recording has been known. However, it complicates the structure of optical pickup apparatus and opposes the demands for compactness and lower cost.
On the other hand, while optical pickup apparatus using an optical element shaped like a parallel plate or the like as an objective lens thereof have recently been known (Japanese Unexamined Patent Publication No. 61-287042, No. 8-62493, No. 7-191219, and the like), they fail to satisfy the demands for using two wavelengths of light for reproducing or recording their corresponding optical recording media having disc thickness values different from each other.